1. Field of the Invention
This invention relates to a method for reforming fats and oils with enzymes, and more specifically relates to a method for reforming fats and oils which comprises treating the fats and oils with two or more kinds of lipases to convert diglycerides and/or monoglycerides (hereinafter these glycerides are collectively referred to as partial glycerides) included in the fats and oils to triglycerides (hereinafter abbreviated as TG).
In other words, the invention relates to a method for reforming fats and oils which comprises converting the partial glycerides contained as impurities in the fats and oils to obtain fats and oils having a high TG content, namely fats and oils having a low partial glyceride content in a high yield.
Enhancement of the TG content in fats and oils leads to solid fats having a high melting point and good quality, and thus contributes to the industry of food such as chocolate.
2. Description of the Prior Art
Fats and oils generally contain esters of fatty acid(s) and glycerol, i.e. TG as a main component, and contain as components other than TG small amounts of partial glycerides, free fatty acids and unsaponifiable matters.
Partial glycerides included in fats and oils include, for example, those synthesized in the living bodies of animals and plants and microorganisms, those produced by partial hydrolysis of TG by the action of lipase contained therein or by a non-enzymatic action during the preservation of the fats and oils, or those produced as by products in the steps of processing of fats and oils such as ester interchange or synthesis of fats and oils from fatty acid(s) and glycerol. In natural fats and oils, palm oil recovered from the sarcocarp of Elaeis guineensis and olive oil recovered from the fruit of Olea europaea have a high water content and are susceptible to hydrolysis with enzymes during the recovering steps and preservation of the fats and oils, and rice bran which is a raw material of rice oil has a strong lipase activity. For the above or other reasons, fats and oils prepared therefrom generally have a high partial glyceride content.
As methods of removing impurities in fats and oils, there have been carried out alkali purification for the purpose of the removal of fatty acids, vacuum steam distillation for the purpose of the removal of odor components and other volatile components and fatty acids, and degumming for the purpose of the removal of gummy matters, carbohydrates, proteins, etc. Removal of these impurities is relatively easy. On the other hand, mutual separation of TG, diglyceride (hereinafter abbreviated as DG) and monoglyceride (hereinafter abbreviated as MG) has been carried out only by means for their analysis, namely by methods such as column chromatography and thin layer chromatography using silica gel as well as gel permeation chromatography utilizing molecular sieve effect. Further, for the purpose of the removal of fatty acids and the fractionation of TGs contained in fats and oils, there have been carried out fractional crystallization utilizing the difference in melting point of TGs, liquid-liquid extraction utilizing the difference in solubility in solvents, fractional distillation utilizing the difference in boiling points and molecular distillation. By these methods, removal of MG was more or less effected but removal of DG was difficult.
Recently, as a method of removing partial glycerides using an enzyme, there has been reported a method of specifically hydrolyzing and removing partial glycerides in fats and oils utilizing a partial glyceride lipase which has no or almost no reactivity with TG (Japanese Patent Unexamined Published Application No. 62-287). On the other hand, there has been reported a method of obtaining fats and oils having a high TG content by ester synthesizing TGs from partial glycerides and fatty acids with triglyceride lipase in the presence of water of 100 ppm or less (J. Am. Oil Chem. Soc. 64(9), 1252(1987).
The removal of partial glycerides from fats and oils by chromatography might be possible in a laboratory scale but could scarcely be adopted in an industrial scale. Further, by the means for the separation of TG components in fats and oils, it was impossible to separate DG and TG from each other since they form an eutectic mixture. Thus, it has hitherto been desired to industrially remove partial glycerides from fats and oils, but there has been no method suitable for the purpose.
Existence of partial glycerides has various undersirable influences on fats and oils. First, partial glycerides have an action of inhibiting the formation of crystalline nuclei of TG. For example, the existence of MG of about 2% or more in palm oil inhibits the growth of crystalline nuclei of TG and the existence of DG of about 13% in the same oil awfully elongates the life time of .alpha.-type crystals. Further, it is generally considered that the existence of DG in fats and oils or fat and oil products inhibits the transformation of TG crystals from .beta.'-type to .beta.-type [Olea giniaux, 29, 421(1974); Oil Palm News, 22, 10-18(1977); J. Sci. Food Agric. 32, 1197(1981); and Fette Seifen An strichm, 85, 64(1983)].
Fruther, DG forms with TG an eutectic mixture which in turn not only makes the separation of these components difficult but also reduces the solid fat index of the fats and oils. As a result, apparent content of high melting TG having relation to crystal becomes lower than the actual content, yield of the solid components is lowered and at the same time a part of the solid components remains in the liquid components, and thus the fractionation of TG becomes incomplete. Further, partial glycerides themselves have an emulsifying action, and thus in the fractionation of TG crystals using a polar organic solvent, lower the separation efficiency.
In fat and oil products such as cacao substitute fats wherein a sharp melting point is required, partial glycerides make the melting point of the products unclear and broad.
For solving this problem, there have been proposed two methods wherein an enzyme is used. One is a method wherein DG is removed with hydrolysis using a partial glyceride lipase and which has an advantage of requiring no complicated reactors. But according to this method, it is impossible to avoid the lowering of yield of purified fats and oils because of hydrolysis.
The other is one wherein partial glycerides and fatty acids are treated with a lipase in the presence of water in a small amount to ester-synthesize TG. This method is excellent in that it is possible not only to reduce fatty acids and partial glycerides as impurities in fats and oils but also to increase the yield of TG. However, in order to put this method in practical use, it is necessary to enhance the ability of an enzyme to be used and to develop a new reactor.